Synthetic Crude Oil

ABSTRACT

A blend suitable for mimicking the behavior of a crude oil/water blend may include a synthetic composition and brine. The synthetic composition may include: (I) at least one base oil component, (II) at least one surfactant, and (III) at least one particulate component having a particle size in the range from 5 to 500 nm.

This invention relates to a fluid which mimics the behaviour of crude oil. In particular, the inventors have found a particular combination of compounds which, when mixed, are able to mimic the properties of crude oil, especially when in a pipeline mixed with water, especially brine. The invention therefore provides a reference fluid to mimic quiescent and flowing properties of crude oil/water systems.

A crude oil is a mixture of hundreds, if not thousands, of compounds, all interacting in order to form a fluid having specific properties. Crude oil may, in simple terms, be described as comprising saturated components, aromatics, resins, and heavy components (asphaltenes), each of which have a specific role in the overall properties of the oil.

Resins and asphaltenes, for example, are known as interfacially active and therefore, both are potential contributors to the interfacial properties of the crude oil.

Some crude oils such as acidic crude oils contain large concentrations of naphthenic acids (also part of the resin component). Naphthenic acids are a class of organic monoacids with the general formula of RCOOH, where R is a cycloaliphatic moiety. In oil-water systems, the acids are distributed between the different phases and play a surfactant role at the water/oil interface due to their partial affinity to the both water and oil.

Asphaltenes are believed to be suspended as microcolloids, nanocolloids or nanoparticles consisting of about 3 nm particles that have important role in the stabilization of water in crude oils. Each particle consists of one or more aromatic sheets of asphaltene monomers, with adsorbed resins acting as surfactants to stabilize the colloidal suspension. Under unfavourable solvent conditions, resins desorb from the asphaltenes, leading to an increase in asphaltene aggregate size and eventually precipitation of large asphaltene aggregates.

The skilled chemist needs to be able to model the behaviour of crude oil wherever the need arises. In particular, it would be valuable to be able to model crude oil behaviour in a pipeline where it will be mixed with water. Whilst crude oil itself could be used, this obviously involves transportation of the crude to wherever it is needed. Also, crude oil is a noxious substance and is ideally avoided as there are issues with toxicity, volatility, storage, and so on. There is also the issue of laboratory safety. It would be a great deal easier if the research petrochemical chemist had at his disposal a composition that mimicked the behaviour of crude oil which could be used for modelling crude oil behaviour and transport properties.

Moreover, crude oils from different fields have different characteristics. Acidic crudes have high acid content but not all crudes are acidic and so on. A synthetic crude oil could be tailored to match the characteristics of crude oil from any field of interest based on the known parameters of the crude oil being modelled.

The use of synthetic oils to mimic liquid flow is not new. Moreover, synthetic oils have being used to mimic crude oil/water mixtures. Simple primitive systems have been tried unsuccessfully in this regard. Generally base oils with the same viscosity, density, and interfacial tension as crude oil have been tried as synthetic oils. These oils are generally chosen according to the physicochemical data of commercial oils. However, flow properties of these synthetic oils are very different from crude oil even though they have the same interfacial tension, viscosity, and density values. Crude oil is a complex fluid and unless the dynamic interfacial behaviour and emulsion properties of the crude oil are also considered, any mimic is unlikely to represent a useful model. In particular, the behaviour of oil in the presence of water must be considered.

Two phase liquid-liquid flow is described as the concurrent flow of two immiscible liquids encountered widely in the petroleum industry during the production and transportation of oil and water. Liquid-liquid flows are realized by the existence of diverse flow configurations and flow patterns or geometrical arrangement of the phases in the pipe when inhomogeneous fluids are flowing together. The flow patterns are different from each other in the spatial distribution and position of the interface, resulting in dissimilar flow characteristics, such as velocities, hold up profiles, and pressure drops. These internal flow structures depend on variables such as pipe geometry, flow rate as well as physicochemical properties of the liquids.

The pressure drop (ΔP) in pipes strongly depends on the flow regime for the two phase liquid-liquid flows and hence the distribution of the liquids in the cross sectional area of the pipe. The flow behaviour of water-oil emulsions depends on, inter alia, the volume fraction and the droplet size distribution of the dispersed phase. Droplet size and distribution of the water-oil emulsions depend on the competition between break-up and coalescence phenomena.

Experimental investigations on liquid-liquid flow are generally based on binary synthetic oils and water. Models reported in the literature are mainly based on the synthetic oil experiments supported by additional field tests. The prediction models are mainly based on the bulk properties of liquids such as density, viscosity, and interfacial tension. However, crude oil systems contain interfacially active substances (resins and asphaltenes) that influence the formation and stability of droplets. Consequently, liquid-liquid flow properties of crude oil systems are generally different from synthetic oil systems even if both systems have similar density, viscosity, and interfacial tension values. The present inventors have realised that modelling interfacial tension and emulsion behaviour is vital to ensure a good mimic of crude oil. Moreover, the overall emulsion properties must be matched. The key factors therefore are: not only the interfacial tension and viscosity but also droplet size and emulsion properties. As previous studies have not been able to mimic these properties, the results are poor.

It is very important to prepare well characterized synthetic oils as mimics for crude oils to apply the synthetic oil based models to field conditions and understand the transformations. The present inventors have, in particular, prepared synthetic oils for mimicking the laminar flow properties of the crude oil and its emulsions, especially in pipelines.

Surprisingly the inventors have discovered how to mimic crude oil in a simple manner. They have found that the role and distribution of saturated components and aromatics in crude oil primarily determines the viscosity and can be mimicked by a base oil/and optional diluent blend having a certain viscosity.

The resins and asphaltenes, which are interfacially active, present in crude, may be mimicked using a surfactant and particulate component. Thus, the heavy components may be mimicked by adding particulates to the synthetic crude to mimic asphaltene behaviour. Thus the role of each of the main classes of compounds within crude oil may be mimicked collectively in a very simple manner.

The invention therefore enables the mimicry of a complex mixture (crude oil), using a simple alternative. It is further important to note that the inventors have found that the properties of the synthetic oil are, to an extent additive—that is the tuning of one parameter, e.g. the resin parameter with a surfactant, does not necessarily influence the other properties of the composition. For example, the addition of myristic acid as oil soluble surfactant has little effect on viscosity of the composition. Their effect is about 0.2-0.3 mPa·s which is negligible. The addition of the particulate component however, increases the overall viscosity and this is taken into account when preparing the synthetic oil.

Surprisingly the inventors have found that it is sufficient to mix only three components to mimic the most important properties of crude oil (which comprises thousands of compounds), in particular from a flow point of view.

Thus, viewed from one aspect the invention provides a blend, e.g. suitable for mimicking the behaviour of a crude oil, comprising brine (e.g. sea water) and a synthetic composition comprising:

(I) at least one base oil component; (II) at least one surfactant; and (III) at least one particulate component having a particle size in the range from 5 to 500 nm.

Ideally, the synthetic composition will additionally contain a diluent to allow formation of a synthetic oil with similar density and viscosity to crude oil.

The amount of water/brine present can be at least 5 wt %, e.g. up to 65 wt %, e.g. 15 to 55 wt %. Alternatively viewed, the amount of water/brine present can be at least 5% (v/v), e.g. up to 65% (v/v), e.g. 15 to 55% (v/v).

Viewed from another aspect the invention provides a water-in-oil emulsion, for example suitable for mimicking the behaviour of a crude oil, comprising water and a synthetic composition comprising:

(I) at least one base oil component; (II) at least one surfactant; and (III) at least one particulate component having a particle size in the range from 5 to 500 nm.

Viewed from another aspect the invention provides use of a blend or emulsion as hereinbefore defined as a mimic for crude oil.

Viewed from another aspect the invention provides a process for preparing a blend suitable for mimicking a crude oil comprising:

(i) determining the viscosity of the crude oil to be mimicked; (ii) combining at least one base oil component, at least one surfactant, at least one particulate component, and optionally at least one diluent;

so as to obtain a composition having a viscosity within 10%, especially within 1% of the viscosity of said crude oil; and

(iii) mixing with brine.

Viewed from another aspect the invention provides a process for preparing a blend suitable for mimicking a crude oil comprising:

(i) determining the viscosity and interfacial tension of the crude oil to be mimicked; (ii) combining at least one base oil component, at least one surfactant, at least one particulate component and optionally at least one diluent;

so as to obtain a composition having a viscosity within 10%, especially within 1% of the viscosity of the crude oil, and an interfacial tension of within 10%, especially within 1% of the interfacial tension of the crude oil; and

(iii) mixing with brine.

Viewed from another aspect the invention provides a process for preparing an emulsion suitable for mimicking a crude oil comprising:

(i) determining the viscosity and interfacial tension of the crude oil to be mimicked; (ii) combining at least one base oil component, at least one surfactant, at least one particulate component, and optionally at least one diluent;

so as to obtain a composition having a viscosity within 10%, especially within 1% of the viscosity of the crude oil and an interfacial tension of within 10%, especially within 1% of the interfacial tension of the crude oil; and

(iii) mixing with at least 5 wt % of water.

Viewed from another aspect the invention provides a method for producing a reference fluid for a crude oil comprising measuring the properties of the crude oil to be mimicked, such properties comprising viscosity, interfacial tension, and emulsion properties, and based on these properties adding an oil, a surfactant, a particulate, and brine such that the reference fluid mimics the crude oil and its emulsion properties.

DEFINITIONS

The term synthetic is used herein to stress that the composition of the invention is formed by blending the necessary components and does not cover, for example, crude oil itself. The components needed are generally commercially available.

It will be appreciated that crude oil is a term of the art and is a product which may or may not contain water. Crude oil obviously mixes with water in the well and as it is extracted but what is extracted is called crude oil. Even crude oil which is transported on tankers, from which water has been nominally removed, still typically contains amounts of water and this oil is still called crude oil. Crude oils could therefore contain from 0.5 to 65 wt % water. The term crude oil is used herein to cover crude oil completely free of water or in which water is present. The term crude oil/water blend simply implies the presence of some water in the crude oil. The invention is suitable for mimicking crude oils of any water content. In general, the crude oils to be mimicked in the present invention will contain some water, typically sea water.

DETAILED DESCRIPTION OF INVENTION Base Oil

The base oil component used in the invention may be a medical oil, mineral oil, or vegetable oil or mixtures thereof. Typically it will have a viscosity range from 30 to 200 mPa·s at 20° C., preferably 50 to 350 mPa·s

Preferred oils include liquid saturated hydrocarbons such as linear or branched alkanes or cycloalkanes. The density of the base oil may be in the range 0.840 to 0.890 g/mL, especially 0.85 to 0.88 g/mL.

The oil may have an Mw of around 300 to 600.

The oil may also be a medical oil such as one derived from rich liquid hydrocarbons. Preferably the base oil is a medical oil, e.g. one based on hydrocarbons. A medical oil is an oil which is very pure and is a term of the art. It should not have any interfacial active compounds and interfacial tension values against water should be around 45 mN/m. Medical oils are thus oils which have been refined several times. In this way they have no interfacially active molecules. This is the best choice to control the reference oil systems.

It is highly preferred if the oil is Primol 352.

The amount of base oil in the composition of the invention should be at least 50% (v/v), (wt/wt), or (wt/v). Preferably there should be at least 75 wt % base oil, more especially 80 wt % base oil in the composition. It will be appreciated that the amount of base oil employed will be tailored to match the desired viscosity.

This component is preferably a liquid.

Diluent

It is preferred if a diluent is used to ensure that the composition of the invention has properties which closely match those of crude oil. The diluent used in the invention may be an aliphatic or aromatic solvent, i.e. a hydrocarbon solvent such as toluene, xylene, alkane, alkene, cycloalkane or cyloalkene or mixtures thereof. Ideally, each component of the diluent will have at least 6 carbon atoms, e.g. 6 to 20 carbon atoms, especially 8 to 15 carbon atoms. Most especially, the diluent will come from the C9-C13 distillate fraction of crude oil.

Preferred options include Exxsol D grade hydrocarbons. These are dearomatised hydrocarbons and offer ideal diluent properties.

The diluent is preferably a free flowing liquid of low viscosity, e.g. less than 10 mPa·s and is therefore distinguishable from the base oil component. It should be non volatile.

Most preferably, the diluent is a C9-13 aliphatic or cyclic hydrocarbon. The amount of oil in the composition of the invention may be from 0 to 50% (v/v), (w/w), or (w/v). Ideally, the diluent forms up to 25 wt % of the composition, e.g. 5 to 20 wt %.

The overall content of diluent and base oil combined in the composition may be at least 90 wt %, such as at least 95 wt %.

This component is preferably a liquid.

Surfactant

The surfactant used in the composition of the invention may be any suitable surfactant but is preferably oil soluble. Preferably it is a saturated or unsaturated fatty acid, organic acid e.g. naphthenic acid or a mixture thereof or salt thereof.

Ideally the surfactant employed has at least 6 carbon atoms, preferably at least 10 carbon atoms. Ideally, the surfactant is a carboxylic acid or salt thereof. The surfactant is preferably a hydrocarbon (other than the COOH group) and is preferably aliphatic, especially saturated.

Surfactants of formula RCOOH where R is a C8-20 alkyl or alkenyl, preferably C10-15 alkyl, or a salt thereof are preferred. The use of myristic acid (C13) is ideal.

The amount of surfactant in the composition of the invention may range from 0.01 to 5 wt %, (v/v) or (w/v). Ideally the amount of surfactant is 0.05 to 4 wt %, especially 0.1 to 2.5 wt %.

This component can be a solid but is preferably a liquid.

Particulates

The particulate component in the synthetic crude oil may be an inorganic particle, preferably hydrophobic particle, or organic particle. It should not dissolve in the composition and must remain in suspension as a particle. Particles may be coated, for example with surfactants or polymers or the like. Examples of particulates include silica, clay minerals, organic latexes, metal salts such as barium sulphate, calcium carbonate, iron oxides, titanium oxide, and colloidal metals. The use of oxides are especially preferred. The use of silica is most preferred.

The particulate material will preferably be hydrophobic. Ideally, silica particles are employed.

The particulates have sizes in the range from 5 to 500 nm, preferably 5 to 50 nm, e.g. 10 to 20 nm. The particles are ideally approximately spherical. The added particles may consist of flocculated particles.

The content of particulates may range from 0.1 to 10% in either (w/w) or (w/v). Preferably, the particulates form 0.1 to 5 wt %, especially 0.5 to 1.5 wt % of the composition.

Thus, viewed from a further aspect the invention provides a synthetic composition comprising:

(I) at least 65 wt % of base oil component; (II) up to 25 wt % of at least one diluent; (III) 0.01 to 5 wt % of at least one surfactant; and (IV) 0.01 to 5 wt % of at least one particulate component having a particle size in the range from 5 to 500 nm.

The combination of the base oil and diluent components are designed to mimic the saturates and aromatic components of crude oil. By ensuring that the combination of the diluent and base oil has a viscosity to match that of the crude oil in question, a synthetic oil which can mimic crude oil can be formulated.

The surfactant component is designed to mimic the resin and acid components of the crude oil. The surfactant component establishes, inter alia, a mimic for the interfacial properties of crude oil. By ensuring that the combination of the diluent, base oil, and surfactant has a viscosity and interfacial tension to match that of the crude oil in question, a synthetic oil which can mimic crude oil can be formulated.

The particulate component is designed to mimic the asphaltenes (heavy components) in crude oil, especially flocculated asphaltenes. This component allows a stabilized emulsion to be formed with water and is also important to mimic the emulsion properties of the crude oil with water. By ensuring that the combination of the diluent, base oil, surfactant, and particulates has a viscosity, static interfacial tension, dynamic interfacial tension, and rheological behaviour (bulk rheology) to match that of the crude oil in question, a synthetic oil which can mimic crude oil can be formulated and which forms similar emulsions with water to crude oil.

Viewed from another aspect therefore the invention provides a blend comprising a composition as hereinbefore defined and water, preferably brine (e.g. 3.5 wt % salt water), e.g. which has a salt content to match sea water. The content of water may be at least 5 wt %, e.g. 10 to 65 wt %, such as 25 to 55 wt %, especially around 50 wt %. It will be readily appreciated that the brine used in the invention can be tailored to have a salt content to match that of the sea water which the crude in question may contain.

Viewed from another aspect the invention provides an emulsion comprising an oil phase formed from a synthetic oil as hereinbefore described and water, e.g. brine (which ideally mimics sea water).

Composition Properties

It is preferred if the composition of the invention has a density within 10% of that of the target crude being mimicked, especially within 5%, most especially within 1%. That density may be in the range 0.825 to 0.925 g/mL, especially 0.855 0.915 g/mL, more especially 900 to 0.915 g/mL. It is not however, essential that the densities of the crude oil and synthetic oil match and in fact the density of the composition may be less than that of crude oil. It is much more important that they present with the same viscosity. Density is measured using a densitometer, e.g. an Anton PAAR DMA 5000 density meter.

It is preferred if the composition of the invention has a viscosity within 10% of that of the target crude, preferably within 5%, especially within 1%. That viscosity may be in the range 60 to 80 mPa·s, preferably 65 to 75 mPa·s (at 20° C.). It is preferred if the viscosity of the synthetic oil is the same as or slightly less than that of the actual crude being modelled, e.g. 0.5 or 1 mPa·s less than the crude. Viscosity can be measured, e.g. using an Anton Paar Physica MCR 301 rheometer following the techniques described below in the examples section. Of course, as long as the same technique is used to measure a parameter in the crude and the mimicking oil then appropriate comparison can be made.

It is preferred if the composition of the invention has an interfacial tension within 10% of that of the target crude, preferably within 5%, especially within 1%. That interfacial tension may be in the range 21 to 25 mN/m at 20° C. Interfacial tension can be measured by Du Noüy ring method described below in the examples section.

The presence of the particulate component ensures emulsion stability. In one embodiment, this can be measured by average droplet sizes. Average droplet sizes in crude oils are typically around 3 to 6 microns, depending on the energy in the mixture. The more the mixture is shaken the smaller the droplets will be. The droplets can be up to and including 100's of micrometers. The emulsion formed with water and the composition of the invention may have a similar average droplet size, especially 4 to 5 microns. Droplet sizes can be measured by a digital video microscope (DVM) method described below in the examples section.

More significantly, the presence of the particulates ensures that emulsion viscosity behaviour is similar to that of crude oil. Without the particulate component, the viscosity behaviour of crude oil emulsions cannot be mimicked. It is the particles that allow the viscosity of an emulsion (like crude oil) to be mimicked.

It is preferred therefore if the viscosity of the synthetic composition of the invention is within 10% of that of the target crude, preferably within 5%, especially within 1% at shear rates between 100 and 1000 1/s at 20° C.

Manufacture of Synthetic Crude Oil

In order to develop a synthetic oil, it is necessary firstly to establish certain properties of the crude oil to be reproduced. Those properties are at least the viscosity and interfacial tension of the crude and ideally also its density. Finally, viscosity properties under shear should be determined.

Ideally therefore, both the density and the viscosity of the crude should be matched. Measurement of the density and viscosity of the crude oil to be mimicked is routine using the methods mentioned above.

In order to mimic the emulsion properties, what is vital is to mimic the viscosity properties of that emulsion. This viscosity can be measured by the rheometer as described below.

The synthetic mimic is prepared by mixing the base oil and any diluent in such a way that the viscosity and optionally density of the crude oil is mimicked. Density is less important for flow properties than the other factors above. Whereas it can be quite close to the oil one wishes to mimic, this is not crucial for flow properties.

The actual blending process is simple and can involve little more than mixing and stirring the appropriate components in the correct amounts.

The base oil/diluent blend can be used to mimic the flow properties of crude oil at that stage. All that is required therefore is to choose an appropriate ratio of base oil to diluent to match the viscosity and optionally the density. The viscosity of the composition before particle addition must be slightly less than the real crude oil, because dispersed amount of particles affects the viscosity.

In order to mimic the interfacial tension of the crude oil, it will be necessary to additionally add the surfactant component, again such that the properties of the crude oil are matched. This can be achieved as described below in the examples.

Addition of an appropriate amount of surfactant helps mimic the interfacial tension of the crude oil due to the high diffusion and adsorption ability of surfactant molecules to the water/oil interface.

At this point therefore, the viscosity (typically slightly less than the target crude oil), density, and interfacial tension of crude oil are mimicked. However, even if both systems have the same values of viscosity, density, and interfacial tension, emulsion properties are different due to the low stabilization ability of the surfactant.

To mimic emulsion properties, a suitable amount of particulates (which are preferably hydrophobic and nanoscale) are added. This can be achieved by mixing the previous components with particulates. In order to form an emulsion, water is also added, typically at high water cuts such as 50 wt %. Rheology experiments can then be used to check emulsion properties at shear rates from 100 to 1000 1/s. The viscosity behaviour under shear can be matched to that of crude oil with addition of appropriate amount of particulates. Rheology experiments are conducted using the protocols described below. The presence of the particulate component therefore allows mimicking of the emulsion properties of crude oil such as viscosity at different water concentrations and shear rates.

Thus, viewed from a further aspect the invention provides a blend formed from (A) a composition comprising:

(I) at least 65 wt % of base oil component; (II) up to 25 wt % of at least one diluent; (III) 0.01 to 5 wt % of at least one surfactant; (IV) 0.01 to 5% wt % of at least one particulate component having a particle size in the range from 5 to 500 nm; and (B) water.

If the stability of the synthetic oil emulsions is being reproduced then the particulate components can then be added in an appropriate amount to reproduce the behaviour of the crude oil being mimicked. The composition can be mixed with water to provide a model of the behaviour of crude oil during its transport through a pipeline.

It will be appreciated that the viscosity of the synthetic oil, and its density are affected by the addition of the further particulates and surfactant. For example addition of particulates to the base oil and diluent increases the viscosity. When choosing the viscosity of the base oil/diluent component therefore account must be taken of the contribution which may be made by other components. It may be necessary to select a viscosity much lower than that of crude oil in order for the final viscosity, once further components are added, to match the crude oil value.

The blend can then be used to study the behaviour of crude oil, in particular in pipes when blended with water, especially sea water (or water containing salt having a similar concentration to sea water). The behaviour of the synthetic oil at different water concentrations can then be investigated for example. Of particular interest is the investigation of laminar flow, as this is the starting point but turbulent flow is also important. By using the invention therefore, the skilled man can predict the behaviour of crude oil and use that knowledge to design appropriately. As all crude oils are different, the ability to model the behaviour of different crudes is vital as each oil well, oil rig, oil pipe and so on might need a particular design to maximise its potential.

The invention will now be further defined with reference to the following non limiting figures and examples. The examples show that the compositions of the inventions can be used to mimic the behaviour of a North Sea crude. The skilled man is able to extend the principles explained in these examples to other crude oils.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. shows flow curves of the crude oil at various temperatures.

FIG. 2. shows flow curves of Primol 352 and Exxsol D60 mixtures for different proportions at 20° C.

FIG. 3. shows zero shear rate viscosities of Primol 352 and Exxsol D60 mixtures for different proportions at 20° C.

FIG. 4. shows interfacial tension (y) of water/synthetic oil interface as a function of myristic acid concentration at 20° C. Synthetic oil: 16.8% (w/w) of Exxsol D60 in Primol 352.

FIG. 5. shows the effect of Aerosil® R104 particle amount (w/v %) on viscosity of synthetic oil at 20° C. Synthetic oil: 1.90% (w/w) of myristic acid was dissolved in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture.

FIG. 6. shows flow curves of the synthetic oil at various temperatures. Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 7. shows zero-shear rate viscosities of the crude and synthetic oil at various temperatures. Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 8. shows flow curves of the crude and synthetic oil emulsions for different volume fraction of aqueous phase at 20° C. Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 9 a-e. shows the flow curves of the crude and synthetic oil emulsions for different volume fraction of aqueous phase at pipeline temperature (40° C.). Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 10. shows droplet size distributions of the crude and synthetic oil emulsions at different volume fractions of aqueous phase. Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 11. compares the pressure gradients for the crude and synthetic oil emulsions for different flow rates and volume fractions at 40° C. Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 12. compares the Reynolds numbers for the crude and synthetic oil emulsions for different flow rates and volume fractions at 40° C. Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 13. compares the friction factors for the crude and synthetic oil emulsions for different flow rates and volume fractions at 40° C. Synthetic oil: 0.90% (w/v) of Aerosil® R104 particles were dispersed in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture, Exxsol D60 and Primol 352 mixture contained 1.90% (w/w) of myristic acid.

FIG. 14. shows droplet size of target crude oil emulsions in pipeline at 40° C. (Measurements were performed with Focused Beam Reflectance Method (FBRM) and FBRM probe was installed to the pipeline).

FIG. 15. shows droplet size of synthetic oil emulsions in pipeline at 40° C. (Measurements were performed with Focused Beam Reflectance Method (FBRM) and FBRM probe was installed to the pipeline).

EXAMPLES Materials and Methods

A North Sea acidic crude oil was used in this study. It was characterized with respect to SARA components, density, acid number, base number, and water content. Firstly, asphaltenes were precipitated in hexane (1:40) and then remaining maltenes were separated using amino and silica chromatography in a fully automated HPLC instrument with a sample capacity corresponding to 0.6 g of crude oil. The SARA weight fractions were gravimetrically determined after solvent removal. The total acid (TAN) and total base (TBN) numbers of the crude oil was determined using ASTM D664 and ASTM D2896 methods.

Primol 352 (density=0.857 g/mL at 20° C.) and Exxsol D60 (density=0.790 g/mL at 20° C.) were provided from Exxon Mobil, USA. Primol 352 is medical grade white oil and it is purified mixture of liquid saturated hydrocarbons. Exxsol D60 is a distillation product of aliphatic and cyclic hydrocarbons.

Myristic acid (tetradeconaic acid) (99.5% pure) is a saturated fatty acid obtained from Acros Organics, USA. Aerosil® R 104 are hydrophobic fumed silica particles provided by Degussa®, Germany. Analytical grade sodium chloride (NaCl) (99.0 pure) was purchased from Aldrich Chemical Company, USA. A Millipore ultrapure water system was used to provide water with a resistivity of 18.3 MΩcm for all experiments. 3.5% (w/v) of NaCl solution was used as aqueous phase for all experiments as a sea water model. All chemicals were used as received without any further purification.

Characterization of Crude Oil and Silica Particles

Table 1 and 2 summarize the physicochemical properties of used crude oil sample and Aerosil® R104 particles through the study. Studied crude oil sample is acidic (2.23 mg KOH/g crude oil) and it has low asphaltene (0.48% w/w) content. Aerosil® R 104 particles are nanoscale (13 nm), spherical, hydrophobic without charge (isoelectric point) when the aqueous phase pH is around 4.

TABLE 1 Physicochemical Properties of Crude Oil Sample Parameter SARA fractionation Saturates (wt. %) 51.8 ± 1.8  Aromatics (wt. %) 37.2 ± 1.2  Resins (wt. %) 6.31 ± 0.46 Asphaltenes (wt. %) 0.48 ± 0.11 Emulsified water (wt. %) 4.05 ± 0.23 Density (20° C.) (g/cm³) 0.903 ± 0.003 Total acid number, TAN (mg KOH/g) 2.23 ± 0.02 Total base number, TBN (mg/g) 1.11 ± 0.01

TABLE 2 Physicochemical Properties of Aerosil ® R 104 Particles. APS Specific Contact (average Particle surface area angle IEP particle size) shape (BET) (m²/g) (θ) (pH) 13 nm Spherical 155 ± 20 106 ± 2 4

Analytical Methods Interfacial Tension Measurements

The interfacial tension experiments were performed by Sigma 70 Tensiometer (KSV Instruments, Finland) incorporated a precision micro balance. A platinum Du Noüy ring with defined geometry and a precision to move the liquid-liquid interface vertically in a thermosttated glass beaker at 20° C. The platinum ring was immersed in 20 mL of the aqueous phase and then equal volume of the oil phase was carefully added to the aqueous phase. The platinum ring was automatically pulled upwards through the interface and the data needed to calculate the interfacial tension were recorded. The tensiometer was calibrated by measuring the surface tension of ultrapure water (72 mN/m at 20° C.) before each interfacial tension experiment. The average of 5 measurements was taken to represent the measured interfacial tension.

Rheology Measurements

An Anton Paar Physica MCR 301 rheometer (Anton Paar, Austria) with concentric cylinder geometry (CC27) was used to investigate the rheological properties of the crude oil, synthetic oil, and their emulsions. The CC27 concentric cylinder has a cup diameter of 28.92 mm and a bob diameter of 26.66 mm. A Peltier chamber (C-PTD 200) coupled with a cover was used to prevent vertical temperature gradients to reduce the influence of environmental conditions. Motor adjustment of the instrument was performed before measurements. Rheological properties of the crude and synthetic oil were investigated at different temperatures ranging from 20 to 60° C. for shear rates ranging from 100 to 1000 (1/s). Rheological properties of emulsions were tested as a function of shear rate ranging from 100 to 1000 (1/s) at 20° C. The average of 3 measurements was taken to represent the measured viscosity for crude oil, synthetic oil, and their emulsions.

Preparation of Emulsions

The emulsions were prepared at the room temperature (23±1° C.) in batches of 50 mL using a variable speed Ika®-Werke Eurostar digital homogenizer (Ika®-Werke Co., Germany). The known amounts of water and oil were sheared together in a homogenizer with 4-bladed propeller stirrer. The speed of the homogenizer was kept constant at 2000 rpm and the shearing was maintained for about 15 minutes. Immediately after homogenization, emulsion conductivities were determined using an InoLab° digital conductivity meter (WTW, Germany). Emulsion type was also inferred by observing what happened when a drop of each emulsion was added to a volume of either pure oil or pure water.

Droplet Size Determination of Emulsions

Visual observation with a microscope is direct and simple way to get the picture of the droplet sizes in an emulsion. A Nikon Eclipse ME 600 digital video microscope (DVM) together with Image Pro Plus 5.0 software was used to acquire pictures of the water droplets in the oil phase. The droplets were counted and the diameter of the droplets calculated. Areas with clusters of droplets together were avoided to acquire a representative set of droplets. Five different pictures were taken for each emulsion. These choices led to a total number of approximately 200 to 300 droplets analyzed for each emulsion. The droplet size distributions of the crude and synthetic oil emulsions were determined for different aqueous phase volume fractions ranging from 0.10 to 0.40.

Laminar Flow of Emulsions in Pipeline

Laminar flow behaviours of the crude and synthetic oil emulsions in pipeline were investigated using a flow rig designed to establish steady flow of the emulsion at constant temperature. It allows measurements of the emulsion flow rate, pressure drop, and temperature due to flow in straight and horizontal section of pipe.

The emulsion was introduced into the system via loading in expansion tank It is then pumped through the coriolis mass flow meter before it enters the heat exchanger. The emulsion then enters the test section (3 m) where the pressure drop and temperature is measured. After the test section the emulsion returns to the expansion tank from where it is pumped again.

The pressure drop measurements for each experimental test point were sampled for a period of 15 minutes. Pressure drop, mass flow rate, and temperature are measured for every 5 seconds. The emulsion was introduced into the test loop and allowed to circulate 1 hour until the desired test temperature (40° C.) and equilibrium were reached. The flow rate was varied by starting from 0.3 m/s and increasing slowly in steps to the 0.7 m/s. For a given flow rate the pressure drop and temperature in the test section were recorded. Minimum 100 data were recorded for the pressure drop and average value of this data set was used for all calculations for each experimental test point. A sample of the emulsion at each volume fraction of aqueous phase was taken during the flow experiments to determine water content. This “flowing” volume fraction of aqueous phase compared to theoretical volume fraction based on how much water added to the oil phase. The water content of the emulsions was determined by Karl Fischer analysis using ASTM method D4377. The densities of emulsions were also determined by Anton Paar DMA 5000 density meter (Anton Paar, Austria) at 40° C.

In-Situ Droplet Size of Emulsions in Pipeline

In-situ droplet size of emulsions was investigated using the focused beam reflectance method (FBRM) using the Mettler Toledo FBRM probe developed by Lasentec®. FBRM probe tip was placed in the pipeline at 45° angle after the test section.

Example 1 Rheology of Crude Oil

FIG. 1 shows the viscosity (mPa·s) as a function of shear rate (1/s) for crude oil sample at different temperatures ranging from 20 to 60° C. The crude oil displays Newtonian behaviour at all temperatures and its viscosity is 70.2 mPa·s at 20° C. This value was used as a reference point to prepare the same viscosity of synthetic oil using the mixture of Primol 352, Exxsol D60, myristic acid, and Aerosil® R104 particles. It is also clear from the FIG. 1 that viscosity of the crude oil decreases with increasing of the temperature.

Preparation of Synthetic Oil

The mixture of Primol 352, Exxsol D60, myristic acid (CH₃(CH₂)₁₂COOH), and Aerosil® R 104 particles was used as synthetic oil for mimicking interfacial tension, viscosity, and stability level of emulsions for the crude oil sample. Preparation procedures of the synthetic oil are given in the following sections.

Example 2 Viscosity Fixation for Synthetic Oil

A mixture of Primol 352 and Exxsol D60 at differing proportions was used to adjust the viscosity of synthetic oil. Flow curves of Primol 352, Exxsol D60, and their mixtures for different proportions (w/w %) at 20° C. are represented in FIG. 2. The figure shows viscosity as a function of shear rate (1/s). It is clear from the FIG. 2 that Primol 352, Exxsol D60, and different proportions of their mixtures are Newtonian fluids. Zero shear rate viscosities of Primol 352, Exxsol D60, and each proportion of their mixtures are also represented in FIG. 3.

The use of 16.8% (w/w) of Exxsol D60 in Primol 352 is used for further study. This has zero shear rate viscosity of 50.5 mPa·s at 20° C. A value lower than that of the crude oil is selected as the addition of particulates increases viscosity.

Example 3 Interfacial Tension Fixation for Synthetic Oil

In this study, myristic acid was chosen as a synthetic surfactant to mimic the interfacial tension of crude oil sample. FIG. 4 shows the interfacial tension value of crude oil against to aqueous phase at 20° C. To mimic the interfacial tension of crude oil (23.0 mN/m), different amount of myristic acid (0.10-2.5 w/w %) were dissolved in the synthetic oil from example 2 (16.8% (w/w) of Exxsol D60 in Primol 352) and then interfacial tension values against the aqueous phase were recorded at 20° C. The interfacial tension decreases as the myristic acid concentration is increased, because a larger amount of myristic acid is adsorbed on the interface and this causes the reduction of the interfacial tension. The ideal amount of myristic acid was found as 1.85% (w/w) in synthetic oil to mimic the interfacial tension of a specific crude oil at 20° C.

A slightly higher amount of myristic acid (1.90% (w/w)) was dissolved in synthetic oil and this amount was kept constant for the further studies.

Example 4 Stability Level Fixation for Synthetic Oil

In this study, nanoscale (13 nm) Aerosil® R 104 particles (fumed, spherical, and hydrophobic silica particles) were used to adjust the stability level of water-in-synthetic oil emulsions due to the low stabilization ability of myristic acid molecules. Different amount of the silica particles ranging from 0.25% to 1.00% (w/v) were dispersed (with vigorous mixing) in synthetic oil (1.90% (w/w) of myristic acid in 16.8% (w/w) of Exxsol D60 and Primol 352 mixture) and then the stability levels of the crude and synthetic oil emulsions were compared with the bottle test for the volume fraction of aqueous phase at 0.50. It was observed that stability levels have approximately same magnitude for the crude and synthetic oil emulsions when the amount of the dispersed silica particles is between 0.75 and 1.00%. Consequently, necessary amount of the dispersed silica particles in synthetic oil was decided in the range of 0.75-1.00% for the further studies.

The effect of the dispersed amount of the silica particles on the viscosity of the synthetic oil was investigated at 20° C. FIG. 5 shows that viscosity of the synthetic oil increases upon the increase of the dispersed amount of the silica particles in synthetic oil and 0.90% (w/v) of the silica particles in synthetic oil gives the approximately same zero shear rate viscosity (70.6. mPa·s) as crude oil (FIG. 1). Furthermore, effect of the 0.90% (w/v) of the dispersed silica particles to the interfacial tension of synthetic oil was also investigated. The interfacial tension of the synthetic oil was measured as 23.36 mN/m and this value is also very close to the crude oil sample (23.01 mN/m). Thus, 0.90 w/v % of the silica particles in synthetic oil was kept constant for the further studies.

The effect of the temperature on the zero shear rate viscosities of the crude and synthetic oil was also investigated at different temperatures. In FIG. 7, one can clearly see that zero shear rate viscosity of the crude and synthetic oil approximately same as a function of temperature ranging from 20 to 60° C. These results indicate that synthetic oil is also capable of mimicking the viscosity of the crude oil sample at various temperatures.

Example 5 Rheology of Crude and Synthetic oil Emulsions

In FIG. 8, the viscosity (mPa·s) versus shear rate (1/s) plots for the crude and synthetic oil emulsions are compared shear rates ranging from 100 to 1000 (1/s) at 20° C. One can clearly see that the viscosity of the emulsions increases with the increase of the aqueous phase volume fraction for the both systems. Furthermore, for all of four aqueous phase volume fractions, the viscosity of the synthetic oil emulsions is close to the viscosity of the crude oil emulsions.

TABLE 3 Physicochemical Properties of Target Crude Oil and Synthetic Oil (After Preparation Steps) Paramater Target Crude Oil Synthetic Oil Density (g/mL) at 20° C. 0.903 ± 0.03 0.861 ± 0.01 Viscosity (mPa · s) at 20° C. 70.48 ± 0.02 70.91 ± 0.01 Interfacial Tension (mN/m) at 20° C. 23.01 23.36

Example 6 Droplet Size of Crude and Synthetic Oil Emulsions

The flow behaviour of crude oil emulsions mainly depends on the volume fraction and droplet size distribution of the dispersed phase. Droplet size and distribution of the emulsions depend on the competition between break-up and coalescence phenomena. Thus, droplet size and distribution is another important parameter for mimicking the flow properties of crude oils using synthetic systems.

The droplet size distributions, acquired by the digital video microscope, in crude and synthetic oil emulsions at different volume fractions of aqueous phase is represented in FIG. 10. The droplet size ranges from 2 μm to more than 30 μm for the both systems but most of the droplets was detected are in the range of 2-14 μm in diameter. Droplets sizes more than 30 μm were not shown in figures due to the low frequency (less then 1%) of these droplets in the emulsions. The size distribution of the crude oil emulsions is slightly narrower then the synthetic oil emulsions but the mimicry is nevertheless striking.

Example 7 Laminar Flow Behaviours of Crude and Synthetic Oil Emulsions

In fluid mechanics, the Hagen-Poiseuille equation is a physical law that gives the pressure drop in fluid flowing through a long cylindrical pipe. The assumptions of the equation are that the flow is laminar, viscous, and incompressible. The Hagen-Poiseuille's equation is given as follows:

$\eta = \frac{\left( {\Delta \; {P/L}} \right)\pi \; D^{4}}{128\; Q}$

where ΔP is the pressure drop (Pa), L is the length of the pipe (m), η is the viscosity of emulsion (Pa·s), Q is the volumetric flow rate (m³/s), D is the inner diameter of pipe (m), π is the mathematical constant.

In fluid mechanics, the Reynolds number is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces. Consequently, it quantifies the relative importance of these two types of forces for given flow conditions. It is also used to characterize different flow regimes, such as laminar and turbulent flow. Laminar flow occurs at low Reynolds number (<2100) where viscous forces are dominant. However, turbulent flow occurs at high Reynolds numbers (>4000) where inertial forces are dominant.

The Reynolds number is generally given as follows:

$N_{Re} = {\frac{\rho \; {DV}}{\eta} = {\frac{VL}{\mu} = \frac{QL}{\mu \; A}}}$

where V is the mean fluid velocity (m/s), L is length of the pipeline (m), η is the dynamic viscosity of fluid (Pa·s), μ is the kinematic viscosity of fluid (m²/s), ρ is the density of the fluid (kg/m³), and A is the pipe cross-sectional area (m²).

In fluid mechanics, the friction factor (f) is a geometric factor and the velocity head, can be described by following equation for the laminar flow regime

$f = \frac{16}{N_{Re}}$

As the volume fraction of aqueous phase increases, one can expect a corresponding increase in the emulsion viscosity and as a consequence the pressure drop through the producing system increases as well. The graphs represented in FIG. 11 shows the pressure gradients (ΔP/L) of the crude and synthetic oil emulsions in pipeline for different aqueous phase volume fractions and flow rates in laminar flow regime at 40° C. It can be seen from the figure that pressure gradient of the emulsions rises with increased the volume fraction of aqueous phase and flow rate for the both systems. The sudden decrease of the pressure gradient is observed for the crude oil emulsions when the aqueous phase volume fraction is greater than the 0.56. This shows the inversion of water-in-crude oil emulsions to oil-in-water emulsions during the flow in pipeline. The inversion point of the synthetic oil emulsions was very close and was observed when the aqueous phase volume fraction is a little higher, around 0.60.

Furthermore, observed pressure gradients for the synthetic oil emulsions are close to the pressure gradients of the crude oil emulsions at all volume fractions of aqueous phase and flow rates. In FIGS. 12 and 13, Reynolds numbers and the friction factors of the crude and synthetic oil emulsions were also compared for different flow rates at 40° C. Reynolds numbers and friction factors for the crude and synthetic oil emulsions are also very close to the each other for all volume fractions of aqueous phases and flow rates. This imply that synthetic oil is capable of the mimicking the laminar flow characteristics (pressure drop, Reynolds number, and friction factor) of the crude oil and its emulsions in the studied experimental conditions.

CONCLUSIONS

It is very important to prepare well characterized synthetic oils for crude oils and their emulsions to apply the synthetic oil based models to field conditions and understand the transformations of the liquid-liquid flows. In this study, well characterized synthetic oil was prepared from the mixture of Primol 352, Exxsol D60, myristic acid, and Aerosil® R 104 particles for mimicking the laminar flow properties of the acidic crude oil and its emulsions in pipeline. The laminar flow experiments indicate that it is possible to mimic pressure drop, pressure gradient, Reynolds number, and friction factor for the acidic crude oils and their emulsions at different aqueous phase volume fractions and flow rates in a pipeline using the commercially available materials above. 

We claim:
 1. A blend suitable for mimicking the behaviour of a crude oil/water blend comprising a synthetic composition and brine, said synthetic composition comprising: (I) at least one base oil component; (II) at least one surfactant; and (III) at least one particulate component having a particle size in the range from 5 to 500 nm.
 2. A blend as claimed in claim 1 wherein the surfactant is oil soluble.
 3. A blend as claimed in claim 1 wherein the particulate component is hydrophobic.
 4. A blend as claimed in claim 1 wherein said blend mimics crude oil under quiescent and flow conditions.
 5. A blend as claimed in claim 1 wherein said composition comprises a diluent.
 6. A blend as claimed in claim 1 wherein said composition comprises: (I) at least 65 wt % of base oil component; (II) up to 25 wt % of at least one diluent; (III) 0.01 to 5 wt % of at least one surfactant; and (IV) 0.01 to 5 wt % of at least one particulate component having a particle size in the range from 5 to 500 nm.
 7. A blend as claimed in claim 1 wherein the amount of water present in the blend is at least 5 wt %, e.g. up to 65 wt %, e.g. 15 to 55 wt % of the blend.
 8. A water-in-oil emulsion suitable for mimicking the behaviour of a crude oil comprising water and a synthetic composition comprising: (I) at least one base oil component; (II) at least one surfactant; and (III) at least one particulate component having a particle size in the range from 5 to 500 nm.
 9. Use of an emulsion or blend as defined in claim 1 as a mimic for crude oil.
 10. A process for preparing a blend suitable for mimicking a crude oil comprising: determining the viscosity of the crude oil to be mimicked; (ii) combining at least one base oil component, at least one surfactant, at least one particulate component, and optionally at least one diluent; so as to obtain a composition having a viscosity within 10%, especially within 1% of the viscosity of said crude oil; and (iii) mixing with brine.
 11. A process for preparing a blend suitable for mimicking a crude oil comprising: determining the viscosity and interfacial tension of the crude oil to be mimicked; (ii) combining at least one base oil component, at least one surfactant, at least one particulate component, and optionally at least one diluent; so as to obtain a composition having a viscosity within 10%, especially within 1% of the viscosity of the crude oil and an interfacial tension of within 10%, especially within 1% of the interfacial tension of the crude oil; and (iii) mixing with brine.
 12. A process for preparing a water in oil emulsion suitable for mimicking a crude oil comprising: (i) determining the viscosity and interfacial tension of the crude oil to be mimicked; (ii) combining at least one base oil component, at least one surfactant, at least one particulate component and optionally at least one diluent; so as to obtain a composition having a viscosity within 10%, especially within 1% of the viscosity of the crude oil and an interfacial tension of within 10%, especially within 1% of the interfacial tension of the crude oil; and (iii) mixing with at least 5 wt % of water.
 13. A method for producing a reference fluid for a crude oil comprising measuring the properties of the crude oil to be mimicked, such properties comprising viscosity, interfacial tension, and emulsion properties, and based on these properties adding an oil, a surfactant and a particulate and brine such that the reference fluid mimics the crude oil. 